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Date:   Tue, 14 Apr 2020 13:36:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     vpillai <vpillai@digitalocean.com>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, fweisbec@gmail.com,
        keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>, Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, aubrey.li@linux.intel.com,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Joel Fernandes <joelaf@google.com>, joel@joelfernandes.org
Subject: Re: [RFC PATCH 03/13] sched: Core-wide rq->lock
Message-ID: <20200414113639.GS20730@hirez.programming.kicks-ass.net>
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 <855831b59e1b3774b11c3e33050eac4cc4639f06.1583332765.git.vpillai@digitalocean.com>
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On Wed, Mar 04, 2020 at 04:59:53PM +0000, vpillai wrote:
> @@ -6400,8 +6464,15 @@ int sched_cpu_activate(unsigned int cpu)
>  	/*
>  	 * When going up, increment the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2) {
>  		static_branch_inc_cpuslocked(&sched_smt_present);
> +#ifdef CONFIG_SCHED_CORE
> +		if (static_branch_unlikely(&__sched_core_enabled)) {
> +			rq->core_enabled = true;
> +		}
> +#endif
> +	}
> +
>  #endif
>  	set_cpu_active(cpu, true);
>  
> @@ -6447,8 +6518,16 @@ int sched_cpu_deactivate(unsigned int cpu)
>  	/*
>  	 * When going down, decrement the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2) {
> +#ifdef CONFIG_SCHED_CORE
> +		struct rq *rq = cpu_rq(cpu);
> +		if (static_branch_unlikely(&__sched_core_enabled)) {
> +			rq->core_enabled = false;
> +		}
> +#endif
>  		static_branch_dec_cpuslocked(&sched_smt_present);
> +
> +	}
>  #endif
>  
>  	if (!sched_smp_initialized)

Aside from the fact that it's probably much saner to write this as:

	rq->core_enabled = static_key_enabled(&__sched_core_enabled);

I'm fairly sure I didn't write this part. And while I do somewhat see
the point of disabling core scheduling for a core that has only a single
thread on, I wonder why we care.

The thing is, this directly leads to the utter horror-show that is patch
6.

It should be perfectly possible to core schedule a core with only a
single thread on. It might be a tad silly to do, but it beats the heck
out of the trainwreck created here.

So how did this happen?